In optoelectronic applications, there is a conversion between an electrical mode and an optical mode of signal communication. In a conversion from the electrical mode, the electrical element is a light source, such as a Vertical Cavity Surface Emitting laser (VCSEL) or a Light Emitting Diode (LED). On the other hand, a conversion to the electrical mode utilizes a detector as the active optical element.
Optoelectronic modules are often used in coupling system components and to provide the mode conversion. The modules are designed to present a relatively small area along a surface of the module that receives optical fibers and to require a relatively small amount of real estate of a printed circuit board against which another surface of the module is seated. This allows a sequence of optoelectronic modules to be seated in a closely spaced arrangement for parallel handling of a large number of optical signals. Typically, each module includes an array of light sources or detectors and a corresponding number of light beams.
Within a single optical module, each beam may follow a direct path between the associated light detector/source and a lens at the input/output surface of the module. Alternatively, the beam paths may have ninety degree turns from the input/output surface to the light detectors or sources. A mirror or array of mirrors may be used to provide the light bending. Regardless of whether the optoelectronic module includes beam bending, the alignment process used in the fabrication of the module plays a key role in achieving the desired performance, yield and cost objectives. In general, the different alignment processes fall within three categories, namely active alignment, visual alignment, and passive alignment.
In an active alignment process, the light source is energized and the coupling between the light source and its intended target is monitored. Specifically, the magnitude of the output from the light source to the target is quantified as alignment adjustments occur. An optical fiber, lens, source, or some combination of components is positioned to achieve maximum coupling efficiency between the light source and its target. This process tends to be slow and relatively costly, because of the requirement of an effective closed loop control system. The system requirements include a set of actuators with sufficient mechanical resolution and stability, an effective peak search algorithm, and instrumentation to enable the monitoring of the coupling efficiency between the light source and its target.
The visual alignment process also functions as a closed loop system, but relies on visual cues (such as fiducials) or on the position of a light beam as monitored through an infrared camera. Thus, unlike the active alignment process, the magnitude of the output is not monitored. The primary drawbacks to visual alignment are that the capital equipment costs escalate rapidly with the required placement accuracy, and the throughput is often comparable to that of the active alignment process.
Passive alignment typically relies upon kinematics. Passive kinematic alignment can be described as alignment achieved by mating elements on the basis of accurately positioned physical features. For example, an optical fiber may be placed into a silicon submount having an etched V-shaped groove. The diameter of the optical fiber and the dimensions of the V-shaped groove are closely matched, enabling the desired positional control of the optical fiber. Other types of kinematic couplings include the use of holes, pins, and the like. The primary advantages of using passive alignment techniques are the reduction in the system investment and the general reduction in process complexity. The primary obstacle is that the inherent part costs quickly escalate as the required accuracy of part features increases. That is, the cost of a module increases as the positional tolerances of the kinematic alignment features become more demanding.